1. Field of the Invention
This invention relates to a position detector used to, for example, position a mask and a wafer on an apparatus for an exposure process for manufacturing semiconductor devices or to measure the accuracy of the superposition of semiconductor patterns.
2. Description of the Prior Art
FIG. 1 shows a conventional position detector based on an optical heterodyne method using a rectilinear diffraction grating as a positioning mark. A beam of light L1 emitted from a two-wavelength rectilinear polarization laser source 1 travels to a diffraction grating 2. Beams of 0th-order diffracted light L2, 1st-order diffracted light L3 and-1st-order diffracted light L4 from the diffraction grating 2 are deflected by a mirror 3. During the passage through an illumination optical system 4, one of these diffracted light beams is cut while the direction of polarization of one of the other light beams is changed by a half-wave plate. The diffracted light beams are then led to a diffraction grating 6 on a mask 5 and a diffraction grating on a wafer 7 at an angle determined by the numerical aperture (NA) of the illumination optical system 8. A first light beam L5 reflected and diffracted by the diffraction grating 6 and a second light beam L6 reflected and diffracted by the diffraction grating 8 travel in the same direction but they are not exactly superposed because the diffraction gratings 6 and 8 are shifted relative to each other in the y-axis direction. The wafer 7 is driven with a wafer stage 11 and a wafer stage drive circuit 12, while the mask 5 is driven with a mask stage 9 and a mask stage drive circuit 10.
A polarization beam splitter 14 splits light beams L5 and L6. Light from light beams L5 and L6 having the same polarization direction is reflected toward a first detection means 13. However, a knife edge 15 prevents light from light beam L6 from reaching first detection means 13. As a result, first detection means 13 only receives light from light beam L5. Similarly, polarization beam splitter 14 transmits light having the same polarization direction toward a second light detection means 16. However, a knife edge 17 prevents light from light beam L5 from reaching second light detection means 16. As a result, second light detection means 16 only receives light from light beam L6. Beat signals of light detected by the first and second detection means 13 and 16 are supplied to a phase difference meter 18, and outputs from the phase difference meter 18 are input to stage drive circuits 10 and 12.
The principle of positioning the mask 5 and the wafer 7 will be described below with reference to FIGS. 2(a) and 2(b) which are sectional and plan views of the diffraction grating 6 and the mask 5. Composite light Em of a-1st-order diffracted light beam having a frequency f1 and a 1st-order diffracted light beam having a frequency f2 is expressed by the following equation (1): EQU Em=A1 exp{i(2.pi.f1.multidot.t-.delta.m)}+A2 exp{i(2.pi.f2+.delta.m)}(1)
where .delta.m=2p.multidot..DELTA.Xm/p (p: the pitch of diffraction gratings 6 and 8) and A1 and A2 are constants determined experimentally. The variable .delta.m depends upon a displacement .DELTA.Xm of the first diffraction grating 6 in the x-axis direction. If the intensity of light Im of the composite light Em is detected, it is expressed by the following equation (2): EQU Im=.vertline.Em.vertline..sup.2 =A1.sup.2 +A2.sup.2 +2A1.multidot.A2 cos{2.pi.(f1-f2)t-2.delta.m} (2)
If the diffraction grating 6 is moved by .DELTA.Xm, the phase of the light beat signal given by the equation (1), i.e., the phase represented by the third term in the equation (2) is changed by 4.pi..DELTA.Xm/p. It is possible to detect the extent of movement .DELTA.Xm of the diffraction grating 6 by detecting this light beat signal. It is also possible to detect the extent of movement .DELTA.Xw in the x direction of the diffraction grating 8 in the same manner.
The light beat signal Iw detected by the second detection means 16 represents the intensity of composite light of a 1st-order diffracted light beam having a frequency f1 and a-1st-order diffracted light beam having a frequency f2. If .delta.w=2.pi..DELTA.Xw/p, the intensity Iw can be expressed by the following equation (3): EQU Iw=A1.sup.2 +A2.sup.2 +2A1.multidot.A2 cos{2.pi.(f1-f2)t-2.delta.w}(3)
The phase difference .DELTA..phi. between the light beat signal detected by the first detection means 13 and expressed by the equation (2) and the light beat signal detected by the second detection means 16 and expressed by the equation (3) is expressed by the following equation (4): EQU .DELTA..phi.=4.pi.(.DELTA.Xm-.DELTA.Xw)/p (4)
Consequently, the phase difference between the mask diffraction light beat signal and the wafer diffraction light beat signal is detected and the mask stage 9 and the wafer stage 11 are moved relative to each other so that this phase difference becomes 0, thereby accurately positioning the mask 5 and the wafer 7.
In a conventional method for actually measuring and evaluating the positioning performance of an apparatus assembled as an exposure apparatus, a fine pattern formed on a mask is superposed and printed on a wafer and a shift of the pattern on the wafer is measured, as described below.
FIGS. 3(a) and 3(b) show a means for measuring only a shift in a direction along the x-axis, i.e., patterns P1 and P2, each of which is, for example, a resist pattern formed by exposure on a wafer. The patterns P1 and P2 form a sub scale, the division of which corresponds to 0.05 .mu.m. The pattern P1 has a pitch of 7.95 .mu.m, while the pattern P2 has a pitch of 8.00 .mu.m. A surface of the wafer is first exposed to light through a first mask (reticle) to form the pattern P1, and the pattern image is developed. Thereafter, resist is applied and a second mask (reticle) having the pattern P2 is aligned on the wafer. The wafer surface is then exposed to form the second pattern P2. The patterns P1 and P2 are thereby superposed as shown in FIG. 3(c). The patterns P1 and P2 are observed by being magnified with a microscope to read the error in superposing the first and second masks (reticles) by alignment.
In the conventional positioning apparatus based on the optical heterodyne method, however, four alignment patterns, i.e., two pairs of rectilinear diffraction gratings on the mask and the wafer are required if there are two positioning direction axes. Also, in the conventional superposition accuracy measuring apparatus using sub scales, it is necessary to measure superposition shifts along the two axes and therefore to separately form, on the wafer, sub scale patterns for measurement in the two directions.